1. Field of the Invention
The present invention relates to a choke coil designed to eliminate noise generated particularly by an electronic device.
2. Description of the Related Art
There are two modes for circulating noise. One is a normal mode (a differential mode) which circulates noise by generating a voltage difference between power supply lines. The other is a common mode which circulates noise by generating a voltage difference between the power supply lines and ground, but without a voltage difference between the power supply lines. The noise current direction of the normal mode is in the same direction as the current direction of the power supply. The noise current direction of the common mode follows a different loop than the current of the power supply. Choke coils are designed to reduce or eliminate these types of noise.
Generally, a common mode choke coil has a slight normal mode leakage inductance and is thus effective to eliminate a normal mode noise to some extent. However, a separate normal mode choke coil is required in the event that a strong normal mode noise occurs.
Some common mode choke coils have a relatively large normal mode leakage inductance. In such a case, a leakage magnetic flux may have a detrimental effect on peripheral circuits. It is thus necessary to provide a magnetic shield around this type of common mode choke coil.
A conventional choke coil by alone is unable to sufficiently eliminate both common and normal mode noises. To this end, two different choke coils, that is, a common mode choke coil and a normal mode choke coil, conventionally needed to be mounted to a printed circuit board. However, two coils occupy more space than the single coil.
Also, the provision of the magnetic shield brings about a rise in the cost of the choke coil.
Accordingly, it is an object of the present invention to provide a choke coil, which is effective to sufficiently eliminate both common and normal mode noises.